Process for the production of ammonium sulphate and elemental sulphur



Oct. 3, 1944.

PROCESS FOR THE PRODUCTION OF AMMONIUM SULPHATE AND ELEMENTAT: SULPHUR Filed 001.. 2. 1941 IMMOMUM 5/Jl/LP/1/7E JOLl/f/0/V MOTHER LIQUOR c'nrs 741 L um H 30 fifreguirer?) PRZSS u VESSEL ELEMENTIL SULPHUR 6 ToRn E Patented Oct. 3, 1944 PROCESS FOR THE PRODUCTION OF AM- MONIUM SULPHUR SULPHATE AND ELEMENTAL Robert Lepsoe, Trail, British Columbia, and Robert Fraser Mitchell, Ross Canada, assignors to T land, British Columbia, he Consolidated Mining and Smelting Company of Canada, Limited,

Montreal, Quebec, ada

Canada, a company of Can- Application October 2, 1941, Serial No. 413,308

7 Claims.

This invention relate to the production of ammonium sulphate and elemental sulphur by the continuous autoxidation, or conversion, of a solution in which ammonium bisulphite i the principal constituent.

It is relatively well known in the art that the autoxidation reactions of sulphurou acid and of its salts are theoretically capable of yielding two commercial products without requiring extraneous energy to be supplied in the form of heat and, on this account, these reactions have been the subject of extensive investigation. The

autoxidation reaction of sulphurous acid and of its salts are expressed in the general equations:

3H2SO3- 2H2SO4+S+H2O+5L000 calories l) and In the processes of the prior art, there have been certain attendant difiiculties which have adversely affected the application of the autoxidation of sulphite solutions, containing initially ammonium bisulphite or ammonium bisulphite and ammonium monosulphite, to commercial scale operations for the production of ammonium sulphate and elemental sulphur. For example, the autoxidation reaction ordinarily proceeds initially extremely slowly but without warning it may proceed with explosive velocity with undesirable, if nothazardouaeifects.

Various methods have been proposed with'a view to overcoming the difllculties involved in conducting this reaction on a commercial-scale. The usual method proposed for overcoming the dimculties associated with the initial sluggishness and the potential violence of the reaction is to conduct the reaction in two or more stages. Thus, the processes of'the prior art, in general, follow the procedure of converting, in a preliminary stage, substantially all the ammonium sulphite content of the solution into ammonium thiosulphate and then into an ammonium polythionate, or mixtures thereof, by reaction with sulphur, the sulphur being in the form of hydrogen sulphide, elemental sulphur or ammonium polysulphide. The reaction of this mixture of ammonium thiosulphate and the ammonium polythionate toform ammonium sulphate and elemental sulphur may then be eflected at a somewhat more uniform rate in a subsequent stage. These methods, employing batch operations, are conducted with poor heat economy and are attended by all the economic disadvantage inherent in processes which must be conducted discontinuously.

We have found that the autoxidation reaction will proceed continuously at a uniform and rapid rate when an ammonium bisulphite solution, which may contain ammonium monosulphite, is fed, in the presence of either or both of the compounds ammonium thiosulphate and an ammonium polythionate into an ammonium sulphate solution containing free sulphuric acid, the temperature of the reaction being maintained preferably above the melting point of sulphur.

A feature of the invention reside in the man ner in which the normal rate of the autoxidation reaction is accelerated and controlled by the presence of free sulphuric acid and of either ammonium thiosulphate or an ammonium polythionate or mixtures thereof. In this manner, the continuity of the operation of th process is ensured, and new economies incapital and operating costs are thereby made possible.

A further feature of the invention resides in the manner in which the heat generated by the autoxidation reaction is available for use within the process.

A full understanding of the operation of the present invention and the various modifications thereof may be had from the following description and the accompanying drawing, which is in the form of a flow-sheet illustrating a preferred embodiment of the process,

In general, the process of the invention includes treating a solution containing a, relatively high concentration of ammonium bisulphite which, for example, may be formed by the absorption of sulphur dioxide in aqueous ammonia. The ammonium bisulphite solution, which may contain ammonium monosulphite, is passed continuously into a pressure vessel, such as an autoclave, wherein it is fed, in the presence of either or both of the compounds ammonium thiosulphate and an ammonium polythioriata'into an ammonium sulphate solution containing free sulphuric acid. The concentration of free sulphuric acid in the pressure vessel and the temperature therein are controlled within preferred limits so that the autoxidation of the solution fed into the pressure vessel results in the rapid production of ammonium sulphate and elemental sulphur.

In the preferred embodiment of the invention illustrated in the accompanying flow sheet, the ammonium bisulphite feed'solution is preferably the product obtained by the absorption of sulphur dioxide in an ammonium bisulphite-ammonium monosulphite absorbent solution. As

It will be seen that hydrogen ion is consumed in Step (a) andthat unless there is sufflclent hydrogen ion concentration (1. e. free acid) initially present and subsequently maintained. the autoxidation reaction, as represented in the above four steps, does not proceed. It will also be seen that if the reaction as represented above, is to proceed, the solution catalyst S203 must be continually supplied. The S203 is consumed in Step (a), is formed in Step and is again consumed in Step (d). It will be clearly understood, of course, that the invention is not to be limited by theoretical considerations as to what the exact mechanism of the various reactions may be, since it is predicated upon discoveries and results entirely independently of theoretical considerations.

In the operation of the present process, we prefer to maintain in the ammonium sulphate solution in the pressure vessel, free sulphuric acid, at a concentration preferably within the range of from to 50 grams per litre. The maintenance of th acidity is controlled by adjusting the constitution of the feed solution, as will be described in detail hereinafter, or by the addition to the pressure vessel of either sulphuric acid or ammonia as required. If there is not suflicient acid in the pressure vessel to enable reaction (a) to proceed, the continuity of the autoxidation reaction will be disrupted and the concentration of the sulphur dioxide in the pressure vessel solutions will rapidly increase while the temperature will decrease. Unless these conditions are promptly rectified, there will be a non-productive period terminating in a sudden increase in temperature accompanied by an instantaneous increase in the gas pressure within the pressure vessel, which may occur with explosive violence.

An excess of acid in the pressure vessel, above that prescribed as the preferred upper limit, would not be detrimental to the operation of the process. The excess acid, however, would eventually require to be neutralized with ammonia to produce neutral ammonium sulphate.

As mentioned above, in the operation of the present process, we also require the presence of thiosulphate during autoxidation. The thiosulphate is most satisfactorily supplied by continuously by-passing a portion of the ammonium bisulphite feed solution, treating this by-passed portion with ammonia to form a solution containing ammonium bisulphite and ammonium monosulphite and converting this solution into ammonium thiosulphate solution, preferably by reaction with elemental sulphur, and then introducing this thiosulphate solution with the feed solution into the pressure vessel.

In the treatment of the by-passed portion of the feed solution with ammonia, we have found that the rate of formation of ammonium thiosulphate is approximately twice as great when the ratio of ammonium bisulphite .to ammonium V monosulphite in the solution is about 1:3 as it is when the ratio is about 1:1. The choice of the ratio of these salts in this'portion of the solution is, however, a matter of operating economics. In determining the quantity of ammonia which is to be added to the by-passed portion of the feed solution to form monosulphite, it must be borne in mind that while it is desirable to employ the high sulphuretting rate associated with the higher concentration of ammonium monosulphite, it is not desirable to have excessive quanttiies of monosulphite present because of its lower solubility in the pressure vessel feed solution and because it necessitates a higher sulphuric acid requirement during the subsequent autoxidation. We have obtained satisfactory results in the sulphuretting step with an ammonium bisulphite-ammonium monosulphite ratio of about 1:2.

The ammonium thiosulphate is formed by agitating the bisulphite-monosulphite solution with granulated or pulverized elemental sulphur, or by grinding lump sulphur with the solution in a bad mill, preferably at ordinary temperatures (20 C. to 30 C.), as shown in the accompanying flow sheet. Other sulphuretting compounds may be used in place of elemental sulphur in this step. For example, compounds such as hydrogen sulphide and ammonium polysulphide may be used, but such compounds may result in the formation of excessive quantities of complex sulphur compounds which will produce undesirable effects in the autoxidation reaction, as will be explained hereinafter.

The resulting thiosulphate solution is continuously withdrawn from the sulphuretting step and is added to the remainder of the original ammonium bisulphite solution which is being continuously fed into the pressure vessel.

The concentration of thiosulphate which we desire to have present in the feed solution to the presure vessel is determined from a consideration of operating economics. For example, when the thiosulphate content of the feed solution is low, the autoxidation reaction in the pressure vessel will be sluggish. On the other hand, if the thiosulphate content of the feed solution is high, the rate of the reaction in the pressure vessel will be increased but it will be necessary to add sulphuric acid to the pressure vessel'to prevent the solution therein from becoming basic.

We have found that the ammonia, combined as bisulphite, should constitute slightly more than 50% of the total weight of ammonia in the pressure vessel feed solution, provided that the feed solution contains thiosulphate but does not contain any polythionates. If the ammonia, combined as bisulphite, is less than 50% of the total weight of ammonia in the feed solution, that is, if the ammonia, combined as monosulphite or thiosulphate or both, constitutes the major portion of the solution, the autoxidation reaction will produce ammonia as follows:

According to Reactions (3) and (4), the solution in the pressure vessel will become basic and the reaction will cease unless sulphuric acid is added.

On the other hand, if the ammonia, which is combined as bisulphite, substantially exceeds 50% of the total weight of ammonia in the feed solution, the autoxidation reaction will produce acid as follows:

According to Reaction the free sulphuric acid produced will accumulate far'beyond the quantity required for sustaining the autoxidation reaction and it will eventually be necessary to neutralize the excess acid with ammonia.

The theoretically prefer red feed solution, therefore, is one which contains ammonium thiosulphate, and in which the ammonia is so proportioned that neither is free sulphuric acid consumed nor is any excessive amount of free acid produced. The reaction of such a feed solution may be represented as follows:

where a is equal to or less than 1. That is, the ammonium monosulphite may be either wholly or partially replaced by ammonium thiosulphate. In the operation of our process, we prefer to adjust the proportions of monosulphite, bisulphite and thiosulphate in the pressure vessel feed solution so that the ratio of the amount of sulphur, combined as monosulphite and bisulphite, to the amount of sulphur, combined as thiosulphate, is within the range of from 1:1 to 2:1. For example, feed solutions within this preferred ratio range may be obtained when from 50% to 30% of the original bisulphite solution is bypassed, neutralized with ammonia to produce the preferred ammonium bisulphite to ammonium monosulphite ratio of 1:2, and treated with elemental sulphur to form thiosulphate as in the prescribed manner.

If polythionates are present in the feed solution, either partially or wholy in place of the thiosulphate, it is diflicult to predetermine a desired sulphur ratio without the knowledge of the extent and form in which the polythionates are present. In this respect, as previously stated, the use of elemental sulphur is preferred in the sulphuretting step for the formation of thiosulphate, inasmuch as the use of elemental sulphur minimizes the possibility of unstable polythionates being formed.

The pressure vessel is maintained substantially filled with a nearly saturated solution of ammonium sulphate containing free sulphuric acid within the preferred range of from to 50 grams of H2SO4 per litre. The solution within the pressure vessel is preferably maintained at a temperature of from 120 C. to 140 C. The pressure required to maintain these temperatures ranges from 30 to 40 pounds per square inch.

We prefer to feed a highly concentrated bisulphite solution into the pressure vessel inasmuch as its autoxidation will produce a concentrated ammonium sulphate solution. Furthermore, by using a highly concentrated feed solution, sufficient heat is evolved in the autoxidation reaction 1 to avoid the necessity of providing extraneous heat to maintain the temperature of the solution in the pressure vessel within the preferred range of from about 120 C. to 140 C., and, further, excess heat from the reaction is available for the ammonium sulphate crystallizing stage of the process. By using concentrated feed. solution, therefore, if the addition of extraneous heat to the crystallizing stage is necessary, the required amount of extraneous heat is reduced to a minimum.

Under the conditions set out hereinabove, we

have obtained very satisfactory results when the bisulphite feed solution is fed into the ammonium sulphate solution at a rate which is within the range of from 5 to litres of feed solution per minute per 1000 litres of ammonium sulphate solution,

The minimum preferred rate of feeding the bisulphite solution is governed only by operating economics. A low rate of feeding necessarily results in the requirement of more equipment and also in a greater heat loss. The maximum preferred rate of feeding the bisulphite solution is governed by the time taken by the sulphur, which is formed, to settle out so that it may be collected. So far as the autoxidation reaction is concerned, the maximiun theoretical rate of flow of feed solution is of the order of 2,000 litres per minute per 1,000 litres of ammonium sulphate solution in the pressure vessel. This rapid rate of feeding, however, would not allow time for the complete separation of sulphur.

To, avoid turbulence, we prefer to feed the bisulphite solution into the pressure vessel below the level at which the ammonium sulphate is withdrawn. The temperature of the solution in the pressure vessel,'under normal operating conditions, is maintained above the melting point of sulphur, so that the sulphur produced by the autoxidation reaction, being in the form of fused prills, settles through the ammonium sulphate solution and forms, in the bottom of the pressure vessel, a bath of molten sulphur. The molten sulphur may be eitherlcontinuously or intermittently withdrawn from the bottom of the pressure vessel.

The ammonium sulphate and the elemental sulphur are separately withdrawn from the pressure vessel at approximately the rate at which the bisulphitesolution is continuously fed into the pressure vessel so that-the volume of solution therein is maintained approximately constant. The ammonium sulphate solution which is withdrawn is substantially saturated and is continuously passed to a crystallizing stage wherein, upon reduction of the pressure, ammonium sulphate crystallizes out and some water evaporates with a simultaneous reduction in temperature.

' The ammonium sulphate crystals are readily separated from the solution and a portion of the ammonium sulphate mother liquor may be returned to the pressure vessel for the purpose of lowering the temperature and diluting the solution therein to prevent premature crystallization.

The following example serves to illustrate the details of operation of the present process.

Ten litres per minute of an original amount of ammonium bisulphite feed solution, flowing at a rate of 29 litres per minute, were by-passed and partially neutralized with ammonia so that the ratio of ammonium bisulphite to ammonium monosulphite was about 1:2. The partially neutralized solution was converted into ammonium thiosulphate by grinding with pulverized elemental sulphur at ordinary temperatures (20 C. to 30 C.), in a ball mill to form 16 litres per minute of thiosulphate solution. The ammonium thiosulphate solution withdrawn from the ball mill was united with the original ammonium bisulphite stream and fed continuously to the pressure vessel at the rate of 35 litres per minute.

Thefeed solution to the pressure vessel contained grams of sulphur per litre, combined as thiosulphate, 147 grams of sulphur per litre, combined as bisulphite and monosulphite, and 147 grams of combined ammonia per litre. Ap-

proximately 3000 litres of substantially saturated ammonium sulphate solution, containingfifl grams of sulphuric acid per litre, was maintained in the pressure vessel, whiletin the lower portion thereoi, beneath the ammoniumsulphate solu- In the pressure vessel feed solution, the ammonia combined asbisulphite was less than 50% of the total ammonia andthe'resulting ammonium sulphate solution normally would have been basic. It was thus necessary to add sulphuric acid continuously to the pressure vessel in order to maintain in the solution therein a concentration of about 50 grams perlitre of free sulphuric acid.

Sulphur was allowed to accumulate in the bottom of the pressure vessel. Ammonium sulphate solution, nearly saturated at 130 C., was continuously removed from the pressure vessel and, upon reduction of the pressure to that of the crystallizing stage, ammonium sulphate crystallized therein, approximately 20% of the water being evaporated with a simultaneous reduction in the temperature of the solution. A portion of the ammonium sulphate mother liquor was returned from the crystallizing stage to the pressure vessel, the rate at which it was returned being such that the temperature of the pressure vessel was kept constant at approximately 138 0., and the effluent solution'from the pressure vessel was maintained slightly below the saturation point. The remaining water in the crystallizing stage wasevaporated by means of extraneous heat.

It will be understood that the spec fic examples set out hereinabove are merely illustrative of the operation of the process. Various changes may be made by those skilled in the art, in the light of the present teachings, to adapt the process to the production of alkali metal sulphates without departing from the spirit of the invention as more particularly defined in the appended claims.

Having thus fully described our invention, what we claim as new and desire to protect by Letters Patent of the UnitedStates is:

l. A continuous process for producing ammonium sulphate and sulphur which comprises continuously supplying in solution form ammonium bisulphite and a lesser amount of at least one compound of the group consisting of ammonium thiosulphate and the ammonium polythionates to a body of a substantially saturated ammonium sulphate solution which contains free sulphuric acid and which is maintained at a temperature above the melting point of sulphur under superatmospheric pressure, thereby continuously effecting autoxidation of sulphur compounds in bisulphite and a lesser amount of at least one compound of the group consisting of ammonium thiosulphate and the ammonium polythionates to a body of a substantially saturated ammonium sulphate solution which contains free sulphuric acid and which is maintained at a temperature above the melting point of sulphur under superatmospheric pressure, thereby continuously efiecting autoxidation of sulphur compounds in the supplied solution to form ammonium sulphate and elementary sulphur, continuously withdrawing ammonium sulphate solution and precipitated sulphur from said body of solution, separately recovering sulphur, crystallizing ammonium sulphate from the withdrawn solution, and returning at least part of the mother liquor to said body of solution.

3. A continuous process for producing ammonium sulphate and sulphur which comprises continuously supplying in solution form ammonium bisulphite and a lesser amount of at least one compound of the group consisting of ammonium thiosulphate and the ammonium polythionates to a body of a substantially saturated ammonium sulphate solution which contains free sulphuric acid and which is maintained at a temperature between 120 C. and 140 C. under superatmosv drawing ammonium sulphate and precipitated the supplied solution to form ammonium sulphate sulphur from said body of solution, separately recovering sulphur, crystallizing ammonium sulphate from the withdrawn solution, and returning at least part of the mother liquor to said body of solution.

4. A continuous process for producing ammonium sulphate and sulphur which comprises continuously supplying in solution form ammonium bisulphite and a lesser amount of at least one compound of the group consisting of ammonium thiosulphate and the ammonium polythionates to a body of substantially saturated ammonium sulphate solution which contains at least 10 grams free sulphuric acid per litre of solution and which is maintained at a temperature above the melting point of sulphur under superatmospheric pressure, thereby continuously efiecting autoxidation of sulphur compounds in the supplied solution to form ammonium sulphate and elementary sulphur, continuously withdrawing ammoniumsulphate solution and precipitated sulphur from said body of solution, separately recovering sulphur, crystallizing ammonium sulphate from the withdrawn solution and returning at least part of the mother liquor to said body of solution.

5. A continuous process for producing ammonium sulphate and sulphur which comprises continuously supplying in solution form ammonium bisulphite and a lesser amount of ammonium thiosulphate, formed by reacting ammonium bisulphite in separate stages with ammonia and sulphur, to a body of substantially saturated amvmonium sulphate solution which containsv i'ree sulphuric acid and which is maintained at a temperature above the melting point of sulphur under superatmospheric pressure, thereby continuously effecting autoxidation of sulphur com-- pounds in the supplied solution to form ammonium sulphate and elementary sulphur, continuously withdrawing ammonium sulphate solution and precipitated sulphur from said body of solution, separately recovering sulphur, crystallizing ammonium sulphate from the withdrawn solution, and returning at least part of the mother liquor to said body of solution.

6. A continuous process for producing ammonium sulphate and sulphur from ammonium bisulphite solution which comprises by-passing a small portion of the ammonium bisulphite solution and treating it with aqueous ammonia to form a solution in which the ratio of ammonium bisulphite to ammonium monosulphite is within the range of from 1:1 to 1:3, reacting the bypassed ammonium bisulphite-ammonium monosulphite solution with sulphur to form a solution containing ammonium thiosulphate, uniting the main body of the ammonium bisulphite solution with the by-passed treated portion thereof and supplying the resulting solution to a body of a substantially saturated ammonium sulphate solution which contains free sulphuric acid and which is maintained at a temperature above the melting point of sulphur under superatmospheric pressure, thereby continuously effecting autoxidation of sulphur compounds in the supplied solution to form ammonium sulphate and elementary sulphur, continuously withdrawing ammonium sulphate solution and precipitated sulphur from said body of solution, separately recovering sulphur, crystallizing ammonium sul- Nil phate from the withdrawn solution, and returning at least part of the mother liquor to said body of solution.

'l. A continuous process for producing ammonium sulphate and sulphur which comprises continuously supplying a solution of ammonium bisulphite and a lesser amount of ammonium thio-- sulphate in which the ammonia combined as bisulphite is maintained in excess of 50% of the total weight of ammonia in the feed solution, to a body of substantially saturated ammonium sulphate solution which contains free sulphuric acid and which is maintained at a temperature above the melting point of sulphur under superatmos pheric pressure, thereby continuously effecting autoxidation of sulphur compounds in the supplied solution to form ammonium sulphate and elementary sulphur, continuously withdrawing ammonium sulphate solution and precipitated sulphur from said body of solution, separately recovering sulphur, crystallizing ammonium sulphate from the withdrawn solution, and returning at least part of the mother liquor to said body of solution.

' ROBERT LEPSOE.

ROBERT FRASER MITCHELL. 

